The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. More specifically, the present invention relates to an imaging lens that is suitable for mounting on a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, and a network camera.
An imaging lens for mounting on a small camera is required not only to be composed of a small number of lenses, but also to have a configuration that can attain high resolution, so as to be compatible with a high-resolution imaging element that has been available in recent years. Conventionally, a three-lens imaging lens configuration has been often used as such an imaging lens. As the resolution of the imaging element has been increased, it is more difficult to achieve satisfactory performance only with the three-lens configuration. In these years, a four-lens configuration or five-lens configuration has being used more frequently.
Among these lens configurations, because of higher design flexibility, the five-lens configuration is expected to be a lens configuration to be used in the next generation imaging lens. As an imaging lens having such the five-lens configuration, for example, an imaging lens described in Patent Reference has been known.
According to Patent Reference, the imaging lens includes in the order from an object side: a first lens that has a convex object side surface and is positive; a second lens that has a shape of a negative meniscus lens directing a concave surface thereof to an image plane side; a third lens that has a shape of positive meniscus lens directing a convex surface thereof to the image plane side; a negative fourth lens that has an aspheric shape on the both sides and has a concave image plane side surface near an optical axis; and a fifth lens that has an aspheric shape on the both sides and is positive or negative.
In the lens configuration, a lower limit of Abbe's number of the first lens, and upper limits of Abbe's numbers of the second and the fourth lenses are set respectively, so as to correct an axial chromatic aberration and a chromatic aberration of magnification and thereby be compatible with high performances of the imaging lens.    Patent Reference: Japanese Patent Publication No. 2007-264180
According to the imaging lens described in Patent Reference, it is possible to attain relatively satisfactory aberrations. Since the total length of the lens system is long, however, it is difficult to attain both miniaturization of the imaging lens and satisfactory aberration correction.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations in spite of a small size thereof.